MPI calls are slow in an OpenMP section - c

Question

I am attempting to write a hybrid MPI + OpenMP linear solver within the PETSc framework. I am currently running this code on 2 nodes, with 2 sockets per node, and 8 cores per socket.
export OMP_MAX_THREADS=8
export KMP_AFFINITY=compact
mpirun -np 4 --bysocket --bind-to-socket ./program
I have checked that this gives me a nice NUMA-friendly thread distribution.
My MPI program creates 8 threads, 1 of which should perform MPI communications while the remaining 7 perform computations. Later, I may try to oversubscribe the sockets with 9 threads each.
I currently do it like this:
omp_set_nested(1);
#pragma omp parallel sections num_threads(2)
{
// COMMUNICATION THREAD
#pragma omp section
{
while(!stop)
{
// Vector Scatter with MPI Send/Recv
// Check stop criteria
}
}
// COMPUTATION THREAD(S)
#pragma omp section
{
while(!stop)
{
#pragma omp parallel for num_threads(7) schedule(static)
for (i = 0; i < n; i++)
{
// do some computation
}
}
}
}
My problem is that the MPI communications take an exceptional amount of time, just because I placed them in the OpenMP section. The vector scatter takes approximately 0.024 seconds inside the OpenMP section, and less than 0.0025 seconds (10 times faster) if it is done outside of the OpenMP parallel region.
My two theories are:
1) MPI/OpenMP is performing extra thread-locking to ensure my MPI calls are safe, even though its not needed. I have tried forcing MPI_THREAD_SINGLE, MPI_THREAD_FUNELLED and MPI_THREAD_MULTIPLE to see if I can convince MPI that its already safe, but this had no effect. Is there something I'm missing?
2) My computation thread updates values used by the communications (its actually a deliberate race condition - as if this wasn't awkward enough already!). It could be that I'm facing memory bottlenecks. It could also be that I'm facing cache thrashing, but I'm not forcing any OpenMP flushes, so I don't think its that.
As a bonus question: is an OpenMP flush operation clever enough to only flush to the shared cache if all the threads are on the same socket?
Additional Information: The vector scatter is done with the PETSc functions VecScatterBegin() and VecScatterEnd(). A "raw" MPI implementation may not have these problems, but its a lot of work to re-implement the vector scatter to find out, and I'd rather not do that yet. From what I can tell, its an efficient loop of MPI Send/Irecvs beneath the surface.